All these regions are extremely dry, and most are extremely hot too, as you can see, these regions include the Sahara, the Gobi desert, Arizona, Nevada, Central Australia, Iran and large chunks of the Middle East. Whether an area is classified as arid or not depends on the evapotranspiration that I mentioned last time, which is the sum of evaporation and transpiration from plants. This image illustrates evapotranspiration:

Evapotranspiration. Image from Wikipedia

All the arid, and semi arid regions have more evapotranspiration than precipitation (fancy word for rain and snow etc, stuff that falls out of the sky). This means these areas are not able to support much plant life, and consequently, the number of animals which can live in the region are limited. There are of course, plants which survive, and indeed thrive in these conditions, and they have some amazing adaptations to allow them to do so, the same for the animals which live in these conditions.

It is often assumed that deserts have no rain at all, but, some have over 25cm a year. The evapotranspiration however, is greater than this.

So, what makes these regions so dry, and mostly hot?

You remember last time I mentioned the ITCZ? Well, arid climates fall around 30°S and 30°N, which is between the first green lines above and below the equator in this image:

Arid regions are located around the bands with an H to the north and south of the Equator (marked by the ITCZ on this image). Image from Wikipedia

As I mentioned in my first post on climate (HERE), the air circulations from the equator to the edge of the tropics are known as Hadley cells. To recap, the warm air rises at the equator, and moves north or south (depending on the hemisphere), as it cools, it begins to descend. This descent occurs around 30°S and 30°N. This air has deposited much of the moisture it picked up at the equator already (mostly over the rainforest regions), so it is much drier when it reaches these latitudes.

For interested geeky parties like me, these latitudes are also known as the Horse Latitudes (Allegedly due to some old naval tradition for running around on a deck of a ship with a horse…). They are characterised by high pressure, which leads to clear, cloudless skies (due to the lack of water vapour in the atmosphere) and very high temperatures. Up here in the North, when we have high pressure it is associated with those lovely (Or not, depending on your perspective) hot summer days, and those crispy cold awesome winter days (Guess which I prefer!). The lack of clouds in these regions means that whilst the days can be scorchingly hot, the nights can be extremely chilly. The highest recorded temperatures are from desert regions, and they can reach over 40°C in summer on a regular basis.

The deserts which are not hot all year round tend to be the ones at a higher altitude, and these occasionally receive snow in the winter, but their summers are still very hot, and they have very little rainfall.

The semi-arid regions are found bordering desert regions, and the main difference between these, and the arid regions is that they receive a little more rainfall, and so are able to maintain a wider variety of plants and animals.

Now, I should mention that whilst these areas are deserts, Antarctica is also one huge desert, but is not on the list of arid regions. This is because Antarctica has a polar climate, which we will get to in a later post..

Next time, we will move onto the climate I am most familiar with, the wet, windy temperate climates, where we will bump into the Jet Stream, the Gulf Stream, and probably a few other streams too! I will try not to make it too long, promise!